1. Field of the Invention
This invention relates to novel synthetic crystalline compositions, e.g. zincosilicates, which contain within their framework 3-membered rings and which have a framework density (FD) of less than 21 tetrahedral atoms per cubic nanometer. The compositions, which exhibit characteristic X-ray diffraction patterns, may be used as ion-exchangers, adsorbents, catalysts, e.g. hydrocarbon conversion catalysts, and catalyst supports.
2. Discussion of the Prior Art
Zeolites and zeolite-like materials comprise a broad range of microporous, crystalline solids whose main uses are as adsorbents, catalysts and ion exchangers. Differences among such materials exist relative to pore size, shape and capacity which result in certain zeolites being particularly suited for particular functions.
The basic building blocks of zeolites are often tetrahedra of SiO.sub.4 and AlO.sub.4, which connect to form complex networks of one-, two- or three-dimensional molecular-sized tunnels. The pores can be used to sort molecules, and their surfaces to catalyze reactions.
Brunner and Meier (Nature, 337, 146 (1989)) show a correlation between the minimum framework density (FD) defined as the number of tetrahedral atoms (T-atoms) per cubic nanometer and the smallest ring size in the framework. Clearly, the FD is related to the void volume of the crystal: as the FD decreases, the void volume or capacity for adsorption increases. The minimum known FD is 12.5 which corresponds to the void occupying just over half the crystal volume. Materials with an FD more than 12 usually contain rings consisting of 4 or more T-atoms (4-membered rings). The exceptions are ZSM-18 which possesses 3-membered rings (see Stephen L. Lawton and Wayne J. Rohrbaugh, "The Framework Topology of ZSM-18, a Novel Zeolite Containing Rings of Three (Si,Al)-O Species," Science, Vol. 247, pp. 1319-1322 (1990)), and lovdarite, disclosed by Merlino, S. in Acta Cryst. A37. C189 (1981), which possesses 3-membered rings, some of which have T-atoms shared by 4-membered rings. The beryllosilicate lovdarite has the empirical formula K.sub.4 Na.sub.12 (Be.sub.8 Si.sub.28 O.sub.72).18H.sub.2 O.
If the correlation between the minimum FD and ring size is correct, then zeolitic materials with 3-membered rings must be synthesized to achieve materials with a larger void volume (higher capacity).
VPI-5, an aluminophosphate, has an 18-membered ring structure with an FD of 14.2, and contains 4-membered rings, see Davis, M. E. et al. Nature 331, 698-699 (1988). This material has a one-dimensional channel system circumscribed by the 18-membered rings. It has been speculated by Meier in New Developments in Zeolite Science and Technology (eds Murakami, X., IIjima, A. & Ward, J. W.) 13-22 (Elsevier, Amsterdam, 1986) that all extra-large ring structures based upon 4-membered rings will have one-dimensional channel systems, which generally exhibit poor molecule flow characteristics. Because pore systems of greater dimensionality are associated with improved molecular flow and hence improved catalyst performance, materials having both extra-large pore structures and multidimensional channel systems are highly desirable. Meier, supra, has speculated that frameworks having multidimensional extra-large channels would require FDs below 12.
The existence of the beryllosilicate lovdarite lends support to the idea that frameworks based upon 3-membered rings can be synthesized. See, M. E. Davis, Nature, 337, 117 (1989). However, the presence of highly toxic beryllium tends to exclude lovdarite from practical applications.
Zincosilicates are known which contain 3-membered rings. Hemimorphite, described by H. F. W. Taylor, Amer. Min., 47, 932 (1962) and willemite, described by C. Hang, M. A. Simonov and N. V. Belov, Soviet Physics-Crystallography, 15, 387 (1970) are examples of such materials. However, these materials do not possess framework densities of less than 21. H. Okaniwa and S. Kasahara in Japanese Kokai Tokkyo Koho 89, 122,916, May 16, 1989; Appl. Nov. 6, 1987 have described a crystalline zinc silicate zeolite having a composition (1.+-.0.3)M.sub.2/n O.aZnO.(1-a) Al.sub.2 O.sub.2.xSiO.sub.2 (0&lt;a.ltoreq.1, (2-a).ltoreq..times..ltoreq.20, M=cation, n=valency of M, and a characteristic X-ray diffraction pattern. Further examples of zincosilicates are set out in U.S. Pat. No. 4,329,328 to McAnespie et al. and U.S. Pat. No. 4,670,617 to DeSimone et al., the contents of which are incorporated herein by reference.